Physical layer loop back method and apparatus

ABSTRACT

The network device includes a transceiver, a pattern generation unit and a pattern recognition unit. The transceiver connects to a communications medium. The pattern generation unit connects to the transceiver. The pattern generation unit is configured to generate a first code word in response to a self-test signal. The pattern recognition unit connects to the communications medium and a network entity. The pattern recognition unit is configured to receive the first code word from the transceiver and to determine whether the first code word includes a loop back pattern. The pattern recognition unit is configured to generate a second code based upon the first code word and to include in the second code word a pattern different from the first code word.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/413,767, filed Sep. 27, 2002. The contents ofthe provisional application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention generally relates to the field of network diagnostictesting. In particular, the present invention is directed to systems andmethods for testing a transmission line in a network such as an Ethernetnetwork.

Recent innovations in the communications industry have promoted theunprecedented growth of the Internet. As an example of this massivegrowth, numerous wide area networks connected to other networktopologies were used to link networks nationally in order to connectnetworks internationally to form the World Wide Web. This massive growthof interconnecting networks has caused many network management issues.One such issue is that network managers for various service providersare now faced with the task of troubleshooting failures that occurwithin devices connected to their network, within components of thenetwork devices, and within the network's transmission lines. Networkmanagers are required not only to detect a failure, but are alsorequired to determine where such failure is located and how to correctthe failure. Many methods have been created to improve the maintenanceof large data networks.

However, Ethernet, which is a popular protocol for local area networks(LAN), currently does not specify any sort of diagnostic test at thephysical level, such as a loop back, because Ethernet was originallydeveloped as a shared protocol. One current testing technique, which isassociated with UNIX, the Internet, and TCP/IP (Transmission ControlProtocol/Internet Protocol), is PING (Packet Internet Grouper). PING isa path loop test transmitted from one CPU back to another CPU. Pingmerely tests an entire path, but cannot determine exactly where thefailure exists or the location where a transmission line is broken. PINGonly indicates that a failure exists. Accordingly, in a switchedEthernet network, there is no current test for directly testing when afailure occurs on a transmission line. Therefore, a need exists for anetwork diagnostic test on the physical layer within switched Ethernetnetworks.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, provided is anetwork device. The network device includes a transceiver, a patterngeneration unit and a pattern recognition unit. The transceiver connectsto a communications medium. The pattern generation unit connects to thetransceiver. The pattern generation unit is configured to generate afirst code word in response to a self-test signal. The patternrecognition unit connects to the communications medium and a networkentity. The pattern recognition unit is configured to receive the firstcode word from the transceiver and to determine whether the first codeword includes a loop back pattern. The pattern recognition unit isconfigured to generate a second code based upon the first code word andto include in the second code word a pattern different from the firstcode word.

According to another embodiment of the present invention, provided is amethod of performing a diagnostic test in a network. The method includesthe steps of providing a transceiver, generating a first code word inresponse to a self-test signal, and receiving the first code word fromthe transceiver at a network device. The method also includes the stepsof determining whether the first code word includes a loop back patternat the network device, generating a second code based upon the firstcode word and including in the second code a pattern different from thefirst code word.

According to another embodiment of the present invention, provided is anetwork device. The network device includes a transceiver, a patterngeneration means and a pattern recognition means. The pattern generationmeans connects to a connection medium. The pattern generation unit isfor generating a first code word in response to a self-test signal. Thepattern recognition means connects to the connection medium and anetwork entity. The pattern recognition means is for receiving the firstcode word from the transceiver and determining whether the first codeword includes a loop back pattern. The pattern recognition means isconfigured to generate a second code based upon the first code word andinclude in the second code a pattern different from the first code word.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will be more readilyunderstood with reference to the following description and attacheddrawings, wherein:

FIG. 1 is a block diagram of a network device according to an embodimentof the invention;

FIG. 2 describes the Open System Interconnection (OSI) model;

FIG. 3 illustrates a block diagram illustrating a relationship betweenthe OSI model and the IEEE reference model;

FIG. 4 shows an exemplary block diagram of an interface between a MAC(Media Access Controller) and a transceiver according to the invention;

FIG. 5 depicts sublayers of the MAC according to the IEEE referencemodel;

FIG. 6 illustrates a block diagram of a switch according to anembodiment of the invention;

FIG. 7A shows an exemplary loop back according to an embodiment of theinvention;

FIG. 7B depicts a diagram of two network switches connected via aphysical medium according to an embodiment of the invention; and

FIG. 8 is a flowchart of a method for performing a physical layer (PHY)loop back self-test according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides, in an example, an Ethernet PHY loop backself-test to diagnose failures or malfunctions that may occur within anetwork transmission line or a device connected to a network.

According to another embodiment, a network device may be an Ethernetswitch, and accordingly, a packet may refer to an Ethernet frame asdefined by IEEE 802.x and as modified herein. ATM (Asynchronous TransferMode) and other fixed cell length systems, as well as other devices anddatagrams may also be within the scope of the invention.

FIG. 1 is a block diagram of an exemplary network according to anembodiment of the invention. The network may include a switched Ethernetnetwork 102, PCs 104 and 110 and switches 106 and 108. PC 104 connectsto switch 106, which is connected to the Ethernet network 102. Althoughthis example describes a network including switches 106 and 108,Ethernet network 102 may include any forwarding device such as aplurality of switches, hubs, routers, and other network devices forrouting packets within the network. PC 110 connects to switch 108, whichis also connected to Ethernet network 102. In a communication sessionbetween PC 104 and PC 110, packets will be routed from PC 104 to switch106. The packets are routed to the Ethernet network 102 through aplurality of devices along a path, which may be predetermined ordynamically determined, to switch 108, and eventually to PC 110.However, under conventional techniques, when a packet destined from PC104 to PC 110 fails to reach its destination, it is very difficult forthe network manager to determine where the failure has occurred. Forexample, if PC 104 pings PC 110, the ping will return (loop back) to PC104 only if the entire path is clear and every device, component, andline along that path is properly functioning. However, if a device,component or line does fail within Ethernet network 102, it becomesdifficult to isolate the failed piece of equipment. In an Ethernetnetwork, in order to determine whether a line connecting two devices isactive and is functioning properly, the invention provides a loop-backfunction at the physical layer. To transmit packets from PC 104 to PC110, the devices may operate according to the OSI seven-layer model.

As shown in FIG. 2, the OSI model defines a stacked-layer communicationprotocol that allows network devices to operate with each other. Theprotocols defined in each layer are responsible for communicating withthe same protocol layer running on an opposite network device. Eachlayer provides services to the layer positioned immediately above,except for the top-level application layer. In other words, althoughthere are) several distinct layers, these layers do blend together. AnOSI model layer typically communicates with three other layers: thelayer below it, the layer above it, and its peer layer on the othernetwork device. Thus, all communications move through the layers toreach the peer layer on the receiving network device.

To better understand how the network operates and interacts with devicesconnected to the network to perform a diagnostic test, first, the OSI7-layer model is described briefly in further detail. FIG. 2 shows theOSI 7-layer reference model from IEEE 802.3×. Physical layer one isprimarily hardware implemented and is concerned with the transmission ofraw bits over communications channels and ensuring that “1's” and “0's”are received correctly. Layer two, the data link layer, is concernedwith ensuring error-free transmission by dividing data into packets orframes and acknowledging receipt of the packets or frames. Layer twogenerally includes the MAC sublayer and the logical link control (LLC)sublayer. Layer two may be implemented by a combination of hardware andsoftware. Layers three through seven are primarily software implementedand are concerned with specific network functions, such as routinginformation and packet congestion control, managing connections betweensenders and receivers, managing user sessions, data compression,transformation, syntax and presentation, and user interface to thenetwork.

Specifically, the network layer handles multiple point-to-point links inthe case where frames or packets are transmitted across multiple linksto reach their destination. In broadcast networks such as Ethernet, aMAC (Media Access Controller) sublayer is typically added to allowmultiple devices to share and contend for the use of the same medium.

While the data link layer is used to control communication between twodevices that are directly connected together, the network layer providesInternet services. These services ensure that a packet reaches itsdestination when traveling across multiple point-to-point links, i.e., aset of interconnected networks joined by routers. The network layerbasically manages multiple data link connections. On a shared LAN (localarea network), packets addressed to devices on the same LAN are sentusing data link protocols, but if a packet is addressed to a device onanother LAN, network protocols such as TCP/IP (Transmission ControlProtocol/Internet Protocol) are used.

The transport layer provides a high level of control for movinginformation between the end systems in a communication session. Thedestination network device may be on the same network or on a differentnetwork. Transport layer protocols set up a connection between a sourceand destination network device and send data in a stream of packets,meaning that each packet is numbered sequentially and constitutes a flowthat can be monitored to ensure proper delivery and identity in theflow. The protocol also regulates the flow of packets to accommodateslow receivers and ensures that the transmission is not completelyhalted if a disruption in the link occurs.

The session layer coordinates the exchange of information betweennetwork devices by using conversational techniques, or dialogs. Dialogscan indicate where to restart the transmission of data if a connectionis temporarily lost, or where to end one data set and start a new one.

Protocols at the presentation layer are for presenting data. Informationis formatted for display or printing in this layer. Codes within thedata, such as tabs or special graphics sequences, are interpreted. Dataencryption and the translation of other character sets are also handledin this layer.

Applications access the underlying network services using definedprocedures in this layer. The application layer is used to define arange of applications that handle file transfers, terminal sessions, andmessage exchange (for example, electronic mail).

FIG. 3 shows a block diagram illustrating a relationship of OSI and IEEEreference models. Shown here, the transmission medium, such as copperwire, optical fiber, coaxial cable, is attached to the physical mediumattachment (PMA). An interface exists between a physical signaling partof the physical layer and the PMA. The physical signaling subsystemsupports the signaling screen between the MAC layer and the PMA. Notethat although considered part of the physical layer, the physicalsignaling (PLS) subsystem function is normally implemented locally tothe MAC function. The MAC sublayer and the physical layer may primarilybe supported directly in silicon, with implementations which arespecific to particular design criteria. The LLC sublayer, which, withthe MAC, forms the data link layer (DLL), is normally implemented insoftware, as are the higher layers (layer three through seven). The PMAis typically implemented by hardware and is often referred to as atransceiver, an MAU (medium attachment unit), or a PHY device.

Ethernet network 102 may use an interface between MAC 402 andtransceiver 406 to determine whether either network device 104 or 110 isoperating properly. FIG. 4 shows an exemplary block diagram of a MACinterface with a transceiver. In particular, the PLS 404 accesses theinterface between the MAC 402 and the transceiver 406. The PLS 404receives and transmits data from the transceiver 406 and also receivescontrol signals from the transceiver 406. The PLS 404 may receive areference clock. The PLS 404 sends and receives data, clock and othersignals to and from the MAC 402. The data sent between the PLS 404 andthe MAC 402 may be in a format, such as a serial data format, compatiblewith the MAC 402 and the higher layers. Accordingly, the PLS 404 encodesand decodes data in order to interface the physical medium to the MAC402. The MAC 402 is also configured to determine whether or not thetransceiver 406 is operational, such as by sending and receiving a testsignal via the PLS 404.

The MAU (transceiver 406) provides the functional, electrical, andmechanical interface between the internal layers of the network deviceand the particular network medium in use. The transceiver 406 mayinclude six primary functions to perform: transmit data, receive data,loop back, collision detection, signal quality error (SQE) test, andjabber protection. The transceiver 406 may be responsible for receivingdata on a Data Out line and forwarding this data over the network. Thetransceiver 406 may provide the necessary drive capability to transmitthe data over the network without modifying the content of the data inany way. Similarly, the transceiver 406 may ensure that data receivedfrom the network is passed to the inner layers (MAC 402) of the networkdevice. The transceiver 406 may perform signal-amplitude and pulse-widthdetection on the received signal to ensure the signal's quality beforeforwarding the unmodified data. The loop back function may include aMAC-to-MAC path loop back function. The transceiver 406 may beresponsible for detecting collisions on the network. If a collisionexists, transceiver 406 may report back to the network device, such asvia signal. The SQE test may be a self-test to ensure that its collisiondetection is working. A jabber mechanism may be provided to prevent asingle node that continues to transmit for excessively long periods oftime from completely utilizing the entire network.

As shown in FIG. 5, the network device may include higher layers,logical link control (LLC) sublayer, the media access control (MAC)sublayer and physical layer technology for Ethernet communications at10megabits per second, 100 megabits per second and 1 gigabit per second.The physical layer may include a reconciliation sublayer RS, which maybe considered part of the MAC or MII (Media Independent Interface) orGMII (Gigabit Media Independent Interface), a physical coding sublayer(PCS), a physical media attachment sublayer (PMA), and a physical mediadependent sublayer (PMD). The PCS may provide the functions of datacoding and decoding, which are usually independent of the physicalmedium used. However, in gigabit Ethernet, the coding scheme used forfiber media and short-shielded cables may be different from that of UTP(unshielded twisted-pair) copper media. The PMA sublayer may performsymbol serialization and deserialization. This converts the data fromthe media format to the MAC format. The PMD layer may perform thefunction of converting signals from a PMA layer into signals appropriatefor the specific media. For example, if the medium is fiber, theelectrical signals may be converted to optical signals and vice versa.The PHY may also include an autonegotiation function for 100 BASE-T or1000 BASE-T. The autonegotiation sublayer may be considered a separatesublayer in the PHY or may be defined as a function block within thePCS.

FIG. 6 shows a simple block diagram of an exemplary packet forwardingdevice, which is shown as a switch according to this embodiment of theinvention. Switch 600 may include subsystems which perform the keyfunctions of a network switch, including a switching fabric subsystem602, DMA interface 614, address resolution logic (ARL) 604, an LED ordisplay interface 608, Management Information Base (MIB) engine 606, andHDLC/SMP interface 618. These functions represent the upper layers ofthe Ethernet switch. Switch 600 may also include a series of PHY devices612 a through 612 h, which may interface with a series of correspondingMAC devices 610 a to 610 h. The PHY devices 610 a through 610 h mayrepresent layer one devices, while the MAC devices 622 a through 622 hmay be representative of layer two devices. Each PHY may includesublayers or function blocks as described above with reference to FIGS.2–5.

To implement a PHY loop back self test in order to test a connectiondirectly between two PHYs, each PHY may be configured to generate andtransmit a unique pattern. This pattern can be generated in response toa control signal from the MAC or directly from the CPU. Because a PHYmay communicate via a frame-based protocol, the pattern may be a setcode that will not be recognized as a frame. In other words, the PHY mayfunction to recognize a frame and transmit that frame to the MAC forswitching or other network processing. Therefore, the pattern generatedmay be a pattern that would not be recognized as a frame by thereceiving PHY. The pattern may be such that it will not be confused withan SFD (start frame delimiter) field or with a packet. The logic togenerate the pattern may be built into the receiving PHY via a circuit,or may be incorporated into the PHY logical sublayers. The receiving PHYmay also include a pattern recognition subsystem, which may beadditional logic built into the PHY sublayers, or an additional circuit.The pattern recognition subsystem may be configured to recognize theunique pattern generated by the transmitting PHY and to handle theunique pattern as a request for a loop back, rather than as a frame.Accordingly, when data is received from a physical media at thereceiving PHY, the pattern recognition subsystem will recognize whetheror not the data is a frame, packet, or a loop back pattern. If a frameis received at the PHY, then the frame may be handled according tostandard Ethernet protocol or layer one protocol. However, if a loopback pattern is received at the PHY, then the pattern recognitionsubsystem automatically recognizes that pattern as a request for a loopback, and the receiving PHY may generate another unique pattern as areturn response and then send the pattern of the return response to thetransmitting PHY. The return response pattern may be generated via anynumber of means, such as by detecting the incoming pattern via registeror FIFO (first-in first-out), calculating a return response that isunique to the incoming pattern and driving the return response back ontothe medium. Accordingly, the pattern recognition subsystem may beconnected to the appropriate sublayer for generating, calculating, anddriving the response pattern back onto the medium, or the PHY mayinclude the appropriate circuitry in order to perform this function.

The loop back pattern may be sent out from a PHY or other network deviceto certify that a cable installation on the network is working properlyor that another device on the network is working properly and isproperly connected to the network. For instance, the loop back patternmay be sent out to troubleshoot a network device failure, a componentfailure or a line failure and determine where such a failure is locatedand how to correct the failure. To perform a diagnostic test in anEthernet network as shown in FIG. 7A, network device 700 may generate aloop back pattern by transmitting a code word (CW1) which is transmittedto network device 710 via a transmission medium 720. The code word (CW1)of the loop back pattern may be configured so that network device 710recognizes that the code word is a testing logic, instead of a regulardata packet. For example, the code word may resemble bits, which arearranged as “010101.” Therefore, instead of further processing ortransmitting code word (CW1) to another device on the network, networkdevice 710 may respond to code word (CW1) by creating a loop 725 andreturning a response, code word (CW2), to indicate that network device710 is operating properly and to indicate the operating parameters andconditions of network device 710. Instead of returning the original codeword (CW1), network device may create a unique pattern for code word(CW2). Code word (CW2) may be, for example, “0111” so that code word(CW2) will not be confused with the original code word (CW1). The reasonfor the unique pattern of code word (CW2) is so that the network is ableto detect and prevent looping from occurring within the network. Loopingis a situation where a code word, pattern, packet or frame travels orcircles endlessly within a looping-fashion on a network. If the codeword is retransmitted back to network device 700 from network device710, when network device 700 receives the original code word, networkdevice 700 may retransmit the original code word back unto the networkcausing the code word to loop within the network. Thus, the inventionmay generate a unique pattern for the return response to indicate thatthe original code word has been received by the intended network device.Thus, the network device 710 may change or modify the original codeword. Namely, not only does network device 710 change the original codeword, it may generate a response pattern that is unique to the originalcode word. If the original code word (CW1) is “010101”, code word (CW2)may be arranged as “0111”, instead of “101010”, which could easily beinterpreted as the original code word. Thus, the invention may create areturn response (CW2) that is unique from the original code word (CW1).

Looping may also occur when the intended network device fails to respondto the loop back pattern due to an error within the network device. Inconducting a diagnostic test of the network device, as the code word istransmitted from network device 700 and travels along the network, eachnetwork device inspects the loop back pattern to determine if the codeword is intended for that particular network device. If so, the intendednetwork device will remove the code word from the network and send areturn response. However, if the loop back pattern is not removed by anetwork device, the loop back pattern may continue to endlessly loop orcircle the network.

Another feature of the invention that mitigates the occurrence oflooping is that the switch may include a timer built in the code word.The timer may be transmitted with the pattern so that if the code wordis not removed by a network device within a predetermined time, once thetime period of the timer expires, a network device may drop or discardthe loop back pattern. According to another embodiment, the inventionmay include a disable transmission to combat looping. A disabletransmission may be transmitted by switch 100 to disable the loop backpattern if the return response is not received from the intended networkdevice within a predetermined time.

FIG. 7B shows a block diagram of two network switches connected viaphysical medium, such as a 10 BASE-T line, which uses two pairs ofwires: one pair for transmission and the second pair for reception.Switch 1 may include certain switch functionality which is shown here asthe upper layers, the MAC, and the PHY. Switch 1 may also include a CPUwhich is connected to the MAC and upper layers for assisting withcertain processing functions. The PHY may include a pattern generationsubsystem and a pattern recognition subsystem, which may be in additionto the PHY sublayers or may be embodied within the PHY sublayers.Accordingly, as part of a self test, or as directed, switch 1 mayinitiate a PHY loop back, generate a pattern as described above, andsend the pattern to switch 2 via the line 750. Upon receiving thepattern, the PHY of switch 2, which may also include a patterngeneration means and a pattern recognition subsystem, may recognize thepattern as the loop back pattern and return a unique return responseback to line 750 to the PHY switch 1. When the PHY switch 1 receives thereturn response pattern, it may recognize that the test has beensuccessful, and communication should be established between Switch 1 andSwitch 2. The test may be initiated by the user via an interface orthrough the CPU. The CPU may communicate with the MAC which sends acontrol signal to the PHY to initiate the loop back test. While the PHYis in loop-back mode, recognition of the pattern may fulfill the testand register a successful signal back to the MAC. Success of the testmay be communicated through an interface on the switch or to the uservia the CPU, via a back office connection or over the network. If thePHY of switch 1 does not receive a return pattern, then the switch maybe configured to time out after a predetermined amount of time haspassed. For example, the MAC may terminate the test by sending atermination signal to the PHY after a period of two seconds has passed.In a situation where a timeout has occurred, they system may determinethat the line has failed or that the PHY and switch 2 has failed. Thisfailed result of the test can be reported to the user via a back officeconnection or via the network from the CPU switch 1.

It is important to note that the PHY loop back pattern, which isgenerated by the PHY, should be a pattern that would not be easilyconfused with an SFD or with a packet. Furthermore, the PHY loop backpattern should not be a pattern which may be easily misrecognized as apacket or an SFD if any noise is present in the system.

FIG. 8 shows a flow chart of a method for performing a PHY loop backself test, according to an embodiment of the present invention. At stepS8-1, the self test may be initiated. As described above, initiation maycome from the CPU or from the MAC. For example, a user may initiate aself test within a network device, such as a switch, by communicatingwith the CPU of the switch in order to request the self test. At stepS8-2, the PHY may generate the pattern. As described above, the PHY mayinclude a pattern generation subsystem, which can generate a uniquepattern which will not recognized as a frame or an SFD. This pattern maybe generated at the PHY and sent onto the medium such as a line orfiber. The PHY may wait a period of time at step S8-3 for a response.After a certain period of time has passed, the test may time out at stepS8-4. If a return response is received before the test times out, thenthe test is determined to be successful. Otherwise, after a time out hasoccurred, the test may be considered to be unsuccessful. At step S8-5,the result of the test may be reported. As described above, the resultmay be reported directly to the user or to the switch interface.

Although the invention has been described based upon these preferredembodiments, it would be apparent to those skilled in the art thatcertain modifications, variations and alternative constructions would beapparent, while remaining within the spirit and scope of the invention.According to one embodiment of the invention, the invention provides anEthernet PHY loop back self-test to diagnose failures or malfunctionsthat may occur within a network transmission line or a device connectedto a network. However, in order to determine the metes and bounds of theinvention, reference should be made to the appended claims.

1. A network device comprising: a transceiver connected to acommunications medium; a pattern generation unit connected to saidtransceiver, said pattern generation unit configured to generate a firstcode word in response to a self-test signal; and a pattern recognitionunit connected to said communications medium and a network entity, saidpattern recognition unit receiving said first code word from saidtransceiver and configured to determine whether said first code wordincludes a loop back pattern, wherein said pattern recognition unit isconfigured to generate a second code based upon said first code word andto include in said second code a pattern different from said first codeword.
 2. The network device as recited in claim 1, wherein said patternrecognition unit generates a return response to said self-test signal.3. The network device as recited in claim 2, wherein said returnresponse indicates the operating conditions of said network entity. 4.The network device as recited in claim 1, wherein the pattern generationmeans is configured to generate a disable code word to deactivate saidfirst code word after a predetermined time has expired.
 5. The networkdevice as recited in claim 1, wherein said first code word includes atimer to instruct said network device to drop said first code word aftera predetermined time has expired.
 6. The network device as recited inclaim 1, wherein the pattern generation unit is configured to generate adisable code word to deactivate said first code word after apredetermined time has expired.
 7. The network device as recited inclaim 1, wherein the network device comprises a switch.
 8. The networkdevice as recited in claim 1, wherein the network device comprises arouter.
 9. The network device as recited in claim 1, wherein the networkdevice comprises a hub.
 10. A method of performing a diagnostic test ina network, the method comprising: providing a transceiver; generating afirst code word in response to a self-test signal; receiving said firstcode word from said transceiver at a network device; determining whethersaid first code word includes a loop back pattern at said networkdevice; generating a second code based upon said first code word; andincluding in said second code a pattern different from said first codeword.
 11. The method as recited in claim 10, wherein said step ofgenerating a code word generates a return response to said self-testsignal.
 12. The method as recited in claim 11, wherein the step ofgenerating a code word further comprises the step of: indicating theoperating conditions of said network entity.
 13. The method as recitedin claim 10, further comprising the step of inserting a timer in saidfirst code word to instruct said network device to drop said first codeword after a predetermined time has expired.
 14. The method as recitedin claim 10, further comprising the step of generating a disable codeword to deactivate said first code word after a predetermined time hasexpired.
 15. A network device comprising: a transceiver means fortransmitting and receiving data; a pattern generation means connected toa connection medium, said pattern generation unit for generating a firstcode word in response to a self-test signal; a pattern recognition meansconnected to said connection medium and a network entity, said patternrecognition means for receiving said first code word from saidtransceiver means and determining whether said first code word includesa loop back pattern, wherein said pattern recognition means isconfigured to generate a second code based upon said first code word andinclude in said second code a pattern different from said first codeword.
 16. The network device as recited in claim 15, wherein saidpattern recognition means generates a return response to said self-testsignal.
 17. The network device as recited in claim 16, wherein saidreturn response indicates the operating conditions of said networkentity.
 18. The network device as recited in claim 15, wherein saidfirst code word includes a timer to instruct said network device to dropsaid first code word after a predetermined time has expired.